Potentiometric and Spectroscopic Studies on Nickel(II) 1,4,7,10-tetraazacyclododecane Complexation and Thermodynamic Aspects

Abstract

The complex formation equilibria of nickel(II) with 1,4,7,10-tetraazacydododecane (cyclam, L) were investigated in 0.1 mol L^-1 ionic strength medium, by KCl at three different temperatures (25°, 35° and 45°C). In this research, potentiometric and spectroscopic titrations were applied that were not performed before in cited studies. The protonation constants of H₂L²⁺ and HL⁺ type ions of cyclen were defined; they were in good agreement with other works. The formation constant of NiH₂L⁴⁺ type complex was determined and thus its occurrence in lower pH range was proved that had not taken place in other Ni(II)-cyclam studies. In higher pH range the existence of NiL²⁺ type complex was identified and its formation constant was defined. It was noticed that the stabilities of either NiH₂L⁴⁺ and NiL²⁺ complexes decrease with temperature. By means of speciation diagram the existences of NiH₂L⁴⁺ and NiL²⁺ complexes were also verified. The overall changes in thermodynamic parameters (ΔH, ΔS and ΔG) associated with NiL²⁺ complex formation equilibrium were evaluated graphically.